Ensure spec link fragments dont break/dangle

[ShadowJonathan]

Together with https://github.com/ruma/ruma/issues/862, we could probably download the referred-to-pages, and do a CSS-select for every id-handle the fragments point to, to see if they're still valid.

In other words, we should probably add the functionality to xtask to download the spec pages, look at the fragments mentioned, and actually ensure those fragments are still present in the page.

This could make refactoring such as switching v1.2 -> v1.3 in the spec links a fairly fearless operation.

[jplatte]

Great idea, we should do this. The odd broken link wouldn't hurt that much, though so I'm fine with this being done post 0.5.0.